Disk processing apparatus for reproducing information from a plurality of optical disks having different recording densities

ABSTRACT

An optical disk apparatus comprises an optical pick-up unit for reproducing information on an optical disk, a first reproducer for processing an information signal obtained by the pick-up unit by one of a waveform slice method and a PRML method, and producing a first reproduction signal, a second reproducer for detecting a minimum value of a level of the information signal and producing a second reproduction signal, and a switch for selecting, as a reproduction information signal, one of the first and second reproduction signals, in accordance with a recording density of the optical disk to be reproduced.

BACKGROUND OF THE INVENTION

The present invention relates to an optical disk apparatus using anoptical disk.

In general, in an optical disk apparatus, a laser beam from a laserdiode is focused by an optical system, and an optical disk is scannedwith the focused beam. Thus, binary data recorded on the optical disk isread.

Normally, when the focused beam is radiated on the optical disk, if theentire spot of the focused beam is located outside the pits, the phaseof all the reflected light is the same. Thus, no decrease occurs in theamount of reflected light due to optical interference. On the otherhand, when a part of the spot of the focused beam is within the pit, aphase difference occurs between the reflected light from the inside ofthe pit and that from the outside of the pit. Consequently, bothreflected light components interfere with each other and the amount ofreflected light decreases. In general, the optical disk apparatus is sodesigned that the amount of reflected light becomes minimum when thecenter of the focused beam spot lands on the center of the pit.Specifically, when the entire of the focused beam spot is locatedoutside the pit as shown in FIG. 22, the level of the reproduced signalis high. When a part of the focused beam spot begins to overlap the pit,the level of the reproduced signal begins to decrease. When the focusedbeam spot is located at the center of the pit, the reproduced signallevel takes a lowest value.

The density in operation of the optical disk apparatus has increasedevery year. One of the techniques for achieving high density is atechnique for reducing the diameter of the focused beam. This requires adecrease in wavelength of a laser and an increase in NA (NumericalAperture) of an objective lens. With the reduction of the diameter ofthe laser beam, information can be reproduced from smaller pits.

Even with the high-density optical disk apparatus wherein the focusedbeam diameter is reduced, however, information needs to be reproducedfrom a low-density optical disk which has already been marketed. In thiscase, as is understood from the relationship between the focused beamand the pit when information is to be reproduced from the low-densityoptical disk by the high-density optical disk apparatus, if the focusedbeam is located at the center of the pit, most of the focused beam islocated within the pit as shown in FIG. 23 and the phase of mostreflected light becomes the same. At this time, a decrease in the amountof reflected light due to interference is small. Specifically, when thefocused beam is located outside the pit, the reproduced signal level ishigh. When the focused beam begins to overlap the pit, the reproducedsignal level decreases. When the focused beam is at the center of thepit, the reproduced signal level increases once again. This phenomenonin which the reproduced signal level increases at the center of the pitis referred to as “rebounding”.

Because of the rebounding, the reproduced signal level increases at thecenter of the pit, too, as shown in FIG. 24. Where the reproducedwaveform in this case is detected by a waveform slice method, erroneousdetection will occur even if the threshold is set at any level. Wheninformation is reproduced from the low-density optical disk by thehigh-density optical disk apparatus, as described above, there is aproblem in that an error occurs in the signal detection result due tothe rebounding.

In such an optical disk apparatus, there is a case where a compatibilitycapable of reproducing information among various types of optical diskshaving different recording densities such as CD (CD-ROM, CD-R etc.), DVDRAM, high density DVD-ROM and high density DVD-RAM of the cominggeneration is required.

Since, however, both the optimum wavelength of a light source and theshape of a light beam spot on the optical disk vary from optical disktype to optical disk type, it is generally difficult to correctlyreproduce information from the plural types of optical disks by anoptical disk drive using an optical head having a single light sourceand a single objective lens. Moreover, it is unfavorable to combine aplurality of light sources and a plurality of objective lenses in orderto allow information from being reproduced from the plural types ofoptical disks having different recording densities because the opticalhead is increased in size and costs.

To resolve the above problem, for example, Jpn. Pat. Appln. KOKAIpublication No. 8-339572 proposes an optical disk drive whose opticalhead is provided with a single light source, a single objective lens,and an opening limitation element having a plurality of openings ofdifferent sizes for limiting an opening of the objective lens to allowinformation to be reproduced from a plurality types of optical diskshaving different recording densities. In this optical disk drive, thediameter of a light beam spot on each of the optical disks is varied bythe opening limitation element in accordance with the size of a pitcorresponding to the recording density of an optical disk thereby toreproduce information from the plurality of types of optical diskshaving different recording densities.

On the other hand, in the conventional optical disk apparatus, in orderto enable information reproduction from a plurality of kinds of opticaldisks with different recording densities, the recording density (size ofpits) of the optical disks is merely considered and the opening size ofan opening limiting element is varied. Since the opening size of theopening limiting element is not set in this apparatus in considerationof the relationship among the recording density of the optical disk, thelight source wavelength and the pit depth of the optical disk, goodreproduction is not achieved in the case of information reproductionfrom an optical disk which does not meet the conditions for the lightsource wavelength and pit depth. For example, the reproduced signalintensity decreases, or asymmetry of reproduced signals increases.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical diskapparatus wherein no error occurs in signal detection even if reboundingoccurs in a reproduced waveform.

Another object of the invention is to provide an optical disk apparatuscapable of exactly reproducing information from a plurality of kinds ofoptical disks with different recording densities, with a structure usinga single light source and a single objective lens.

The present invention provides an optical disk apparatus forreproduction, comprising a pick-up unit which reads information from anoptical disk, the information being recorded on the optical disk in aform of pits, and produces an information signal by photoelectricconversion, a first reproducer which processes the information signal byone of a waveform slice method and a PRML method, and produces areproduction signal, a second reproducer which detects a minimum valueof a level of the information signal and producing a reproductionsignal, and a switch which selects, as a reproduction informationsignal, one of the reproduction signal produced by the first reproducerand the reproduction signal produced by the second reproducer, inaccordance with a recording density of the optical disk to bereproduced.

With the above structure, information can be reproduced from alow-density optical disk even with use of a high-density optical diskapparatus wherein a focused beam size is reduced.

This invention also provides an optical disk apparatus comprising apick-up unit including a light source, an objective lens which focuses alight beam emitted from the light source onto an optical disk, and aphoto-detector which detects reflection light from the disk, the pick-upunit reproducing information recorded on the disk along a predeterminedtrack, and an opening limiting unit which limits an opening of theobjective lens such that numerical apertures NAlr and NAlt in a diskradial direction and a track tangential direction of the objective lensin a case where the optical disk is a first disk optimized with respectto a wavelength λ1 of the light source, and numerical apertures NA2r andNA2t in the disk radial direction and the track tangential direction ofthe objective lens in a case where the optical disk is a second diskoptimized with respect to a wavelength λ2 (λ1<λ2) of the light source,satisfy formulae:${{0.95 \cdot \frac{\lambda 1}{\lambda 2} \cdot {NA}}\quad 1r} \leqq {{NA}\quad 2r} \leqq {{1.1 \cdot \frac{\lambda 1}{\lambda 2} \cdot {NA}}\quad 1r}$

 0.95·NAlt≦NA2t≦1.1·NAlt

The present invention also provides an optical disk apparatus comprisinga pick-up unit including a light source, an objective lens which focusesa light beam emitted from the light source onto an optical disk, and aphoto-detector which detects reflection light from the disk, the pick-upunit reproducing information recorded on the disk along a predeterminedtrack, and an opening limiting unit which limits an opening of theobjective lens such that numerical apertures in a disk radial directionand a track tangential direction of the objective lens are made equal ina case where the optical disk is a first disk having a predeterminedrecording density and optimized with respect to a wavelength λ1 of thelight source, and the numerical aperture in the disk radial direction ofthe objective lens is made less than that in the track tangentialdirection of the objective lens in a case where the optical disk is asecond disk having a recording density lower than the first disk andoptimized with respect to a wavelength λ2 (λ1<λ2) of the light source.

Furthermore, this invention provides an optical disk apparatuscomprising a pick-up unit including a light source, an objective lenswhich focuses a light beam emitted from the light source onto an opticaldisk, and a photo-detector which detects reflection light from the disk,the pick-up unit reproducing information recorded on the disk along apredetermined track, and an opening limiting unit which limits anopening of the objective lens such that a beam spot shape on the opticaldisk is the same in a disk radial direction and a track tangentialdirection of the optical disk in a case where the light source has awavelength λ1 and the optical disk is a first disk having apredetermined recording density, and the beam spot shape on the opticaldisk is large in the disk radial direction and small in the tracktangential direction in a case where the optical disk is a second diskhaving a recording density lower than the first disk and optimized withrespect to a wavelength λ2 (λ1<λ2) of the light source.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 schematically shows a structure of an optical disk apparatusaccording to an embodiment of the present invention;

FIG. 2 is a block diagram showing a second reproducer used in theoptical disk apparatus shown in FIG. 1;

FIG. 3 is an operational waveform diagram in a case where the presentinvention is applied;

FIG. 4 is an operational waveform diagram in a case where the secondreproducer including a correction circuit is used;

FIG. 5 is a block diagram of the second reproducer including thecorrection circuit;

FIG. 6 is a circuit diagram of a correction circuit which increases acontinuous length of code bits “1” by one pit;

FIG. 7 is a circuit diagram of a correction circuit which increases acontinuous length of code bits “0” by one pit;

FIG. 8 shows a reproduced waveform in a case where there is a trackingoff-set;

FIG. 9 shows a first correction example of a very small portion due todeformation of a spot shape;

FIG. 10 shows a second correction example of a very small portion due todeformation of a spot shape;

FIG. 11 shows an example wherein tracking cannot be performed with anormal spot shape;

FIG. 12 shows improvement of tracking performance due to deformation ofa spot shape;

FIG. 13 schematically shows a structure of an optical disk apparatusaccording to a second embodiment of the invention;

FIGS. 14A, 14B and 14C show specific structures of the opening limitingelement used in the present invention;

FIG. 15 is a view for explaining the definitions of a modulation degreeand asymmetry of reproduced signals from the optical disk;

FIG. 16 shows a reproduced signal waveform in a case where informationis reproduced from a first disk by a first optical disk apparatus;

FIG. 17 shows a reproduced signal waveform in a case where informationis reproduced from a second disk by a second optical disk apparatus;

FIG. 18 shows a reproduced signal waveform in a case where informationis reproduced from the second disk by the first optical disk apparatus;

FIG. 19 shows a reproduced signal waveform in a case where informationis reproduced from the second disk by the first optical disk apparatuswith use of an elliptic opening having a numerical aperture of 0.36 in aradial direction of the disk and a numerical aperture of 0.36 in atangential direction of the track;

FIG. 20 is a view for describing the optical disk apparatus according tothe another embodiment of the invention and specifically shows an NAdependency characteristics of asymmetry of reproduced signals from theoptical disk;

FIG. 21 shows a reproduced signal waveform in a case where informationis reproduced from the second disk by the first optical disk apparatuswith use of an elliptic opening having a numerical aperture of 0.36 in aradial direction of the disk and a numerical aperture of 0.6 in atangential direction of the track;

FIG. 22 shows a conventional state of the light beam focused on theoptical disk;

FIG. 23 shows a state of the focused light beam when rebounding occurs;and

FIG. 24 shows an operational waveform diagram in a case where therebound occurs.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will now be described withreference to the accompanying drawings.

According to an optical disk apparatus shown in FIG. 1, a read signal(READ signal) is input to a laser diode driver (LD driver) 2 of apick-up unit to drive a laser diode 3, whereby information recorded onan optical disk 1 is read out. Specifically, if the laser diode 3 isdriven, a laser beam is output from the laser diode 3 and is focused onthe optical disk 1 through a collimate lens 4, a beam splitter 5 and anobjective lens 6. Reflected light from a pit stream on the optical disk1 is reflected by the beam splitter 5 and made incident on a focusinglens 7. The focusing lens 7 focuses the reflected light on aphoto-detector 8.

The photo-detector 8 outputs an electric signal, which corresponds tothe incident reflected light, to a first reproducer 10 via an amplifier9 of a signal processing unit. The first reproducer 10 processes theamplified signal by means of a wave slice method or a PRML (PartialResponse Maximum Likelihood) method, thereby producing a reproducedsignal.

In this invention, a second reproducer 11 is provided in parallel to thefirst reproducer 10 in rear of the amplifier 9. A switch 12 is providedin rear of the first and second reproducers. The switch 12 is operatedto select one of the outputs from the first and second reproducers 10and 11 in accordance with the kind of the reproduced optical disk. Forexample, the optical characteristics of the optical disk, e.g. CD or DVD(particularly high density DVD), are determined on the basis of anoutput signal from the photo-detector 8, and the switch 12 is operatedin accordance with the determination result. Specifically, it isdetermined whether the reproduced signal is being normally reproduced,and if the reproduced signal is detected as noise, the switch 12 ischanged over. This operation of the switch is effected by a controllerprovided on the disk apparatus, although not shown.

In the above structure, if the output of the amplifier 9, i.e. theamplified reproduced signal, is supplied to the second reproducer 11, aminimum value of the reproduced signal level is detected by the secondreproducer 11 and one of the outputs of the first and second reproducers10 and 11 is selected by the switch 12 in accordance with the kind ofthe reproduced optical disk.

The structure and operation of the second reproducer of the presentinvention will now be described with reference to FIGS. 2 and 3.

As is shown in FIG. 2, the second reproducer 11 comprises a differentialcircuit 13, a rising zero-cross detector 14, a rising detector 15 and aPWM (Pulse Width Modulation) circuit 16.

In the second reproducer 11 with the above structure, if the reproducedsignal is input to the differential circuit 13, the waveform of thereproduced signal is differentiated. The differentiated waveform, asshown in FIG. 3, zero-crosses at extreme values of the reproducedwaveform. If the differentiated waveform is input to the risingzero-cross detector 14, the rising zero-cross detector 14 detects pointswhere the polarity of the differentiated waveform changes from negativeto positive, and outputs detection pulses to the rising detector 15. Therising detector 15 detects the rising edge of the pulse and sets a codebit “1” at the detection point and a code bit “0” at other points. Ifthe output signal of the rising detector 15 is input to the PWM circuit16, the PWM circuit 16 subjects the output signal of the rising circuit15 to PWM and thus produces decode data.

The bit stream after PWM does not necessarily coincide with the recorddata. For example, as shown in FIG. 4, if the bit stream after PWM iscompared with the record data, coded bit “0” appears at the rear end ofa train of successive coded bits “1”. Reversely, coded bit “1” mayappear at the rear end of a train of successive coded bits “0” (notshown). It can be estimated from the specifications of the optical diskto be reproduced what difference arises between the stream after PWM andthe record data. If a correction circuit 17 is provided at the rearstage of the PWM circuit 16, as shown in FIG. 5, decoding can becorrectly performed.

For example, when the bit stream after PWM conversion is compared withthe record data, if the coded bit “0” appears at the rear end of thetrain of successive coded bits “1”, the correction circuit 17 comprisinga delay circuit 18 and an OR circuit 19, as shown in FIG. 6, is providedat the rear stage of the PWM circuit 16 to perform data correction.Thus, decoded data coinciding with the record data can be obtained.Specifically, if coded bit “1” is input to the delay circuit 18 andinput to the OR gate 19 with a delay of a one-bit period, coded bit “1”is output from the OR gate even if the next PWM coded bit becomes “0”.Accordingly, corrected data corresponding to the record data isobtained.

On the other hand, when the bit stream after PWM conversion is comparedwith the record data, if the coded bit “1” appears at the rear end ofthe train of successive coded bits “0”, the correction circuit 17comprising a delay circuit 17 and an AND circuit 20, as shown in FIG. 7,is provided at the rear stage of the PWM circuit 16 to perform datacorrection. Thus, decoded data coinciding with the record data can beobtained. Specifically, if coded bit “0” is input to the delay circuit17 and input to the AND gate 20 with a delay of a one-bit period, codedbit “0” is output from the AND gate even if the next PWMs coded bitbecomes “1”. Accordingly, corrected data corresponding to the recorddata is obtained.

In the above embodiment, a filter for emphasizing an amplitude of adifferential waveform may be provided in a case where the rebound issmall and the amplitude of the differential waveform obtained from thedifferential circuit 13 of the second reproducer 11 is small. Thereby, asignal with a small rebound can be reproduced with high precision.

FIG. 8 shows a variation in a reproduced waveform due to off-track whena rebound is occurring. The rebound amount obtained when the laser beamspot scans a first scan line 1 extending through the center of a pit 21,that is, a reproduction signal level 22, is greater than the reboundamount obtained when the spot scans a second scan line 2 extending offthe center of the pit 21, that is, a reproduced signal level 23corresponding to the rebound amount due to off-track. Specifically, therebound amount due to off-track is less than that due to on-track. Thefocused beam can thus scan the track center if a tracking control isperformed so that the reproduced signal level may take a maximum valueat the center of the pit on the basis of the reproduced waveform. Thistracking control can be achieved by delivering the reproduced signal dueto on-track to the tracking control circuit of the optical diskapparatus.

When the signal detection is effected by detecting the minimum value ofthe reproduced signal, the location of the minimum value is notnecessarily at the optimal point. For example, as shown in FIG. 9, theminimum value of the reproduced signal obtained with a normal spot(substantially circular) is displaced outside the optimal point. In thiscase, by changing the shape of the spot to a vertically elongatedelliptic shape, the location of the minimum value is shifted to theinside and made closer to the optimal point. Inversely, in FIG. 10, theminimum value of the reproduced signal obtained with the normal spot(substantially circular) is displaced inside the optimal point. In thiscase, by changing the shape of the spot to a horizontally elongatedelliptic shape, the location of the minimum value is shifted to theoutside and made closer to the optimal point.

As has been described above, when the location of the minimum value ofthe reproduced signal is displaced from the optimal point, the shape ofthe spot is changed so that the location of the minimum value may becloser to the optimal point.

In a system wherein the tracking is made to increase the rebound amountto a maximum, there is a case where optimal tracking cannot be performeddue to the relationship between the pit and the spot. For example, in acase illustrated in FIG. 11, all the beam spot resides on a bottomportion of the pit near the center of the pit, irrespective of the scanlines 1 and 2. In this case, the rebound amount of the reproduced signalis equal when the spot scans the scan lines 1 and 2.

In order to solve this problem, the spot shape is changed to avertically elongated elliptic shape, as shown in FIG. 12. By changingthe spot shape to the vertically elongated elliptic shape, all the spotenters the bottom portion of the pit when the spot scans the scan line 1and part of the spot overlaps a wall portion of the spot when the spotscans the second scan line 2. That is, the difference in rebound amounttakes a maximum value only when the spot scans the scan line 1, andexact tracking can be effected.

The mechanism for changing the beam spot shape, as described above, canbe realized by inserting an opening limiting element between theobjective lens and the beam splitter as described hereinafter.

As has been described above, according to the present invention, thelocation of a negative extreme value is detected from the reproducedwaveform with the rebound. The coded bit at the detected location isdetermined to be “1” and the coded bit at other locations is determinedto be “0”. Subsequently, the result of determination is subjected to PWMso that no error may occur in signal detection even when the reboundoccurs in the reproduced waveform.

An optical disk apparatus according to a second embodiment of theinvention will now be described with reference to FIG. 13. In FIG. 13,an optical disk 110 is, for example, a read-only optical disk andcomprises a transparent substrate having pit streams representing recordinformation, a reflection layer or a recording layer (hereinafterreferred to as “reflection/recording layer”) 111 formed on thetransparent substrate, and a protection layer formed on thereflection/recording layer 111. Where the reflection/recording layer 111is a recording layer, it may be formed of any material capable ofrecording information with radiation of a light beam. For example, aphase change medium layer or a photo-magnetic layer may be used as thereflection/recording layer 111.

When information is to be reproduced from the optical disk 110, the disk110 is rotated by a spindle motor 112 driven by a motor driver 113 and alaser diode (LD) 115 is driven by an LD driver 114 to emit a light beam.The light beam emitted from the semi-conductor laser 115 is converted toa collimated light flux through a collimator lens 116 and the collimatedlight flux is made incident on the objective lens 119 through a beamsplitter (half prism) 117 and an opening limiting element 118 (to bedescribed later in detail). The beam is then focused on thereflection/recording layer 111 of optical disk 110 by the objective lens119, and a small beam spot is formed on the reflection/recording layer111.

The reflected light from the reflection/recording layer 111 of disk 110is returned through the objective lens 119 and opening limiting element118 in a direction reverse to the direction of the incident beam on thedisk 110. The reflection light is guided to a focusing lens 120 via thebeam splitter 117 and then focused on a photo-detector 121 by thefocusing lens 120. The photo-detector 121 is a plural-segment splitphotodetector having a light-receiving surface divided into pluralsegments, e.g. two segments or four segments (i.e. two- or four-segmentsplit photo-detector). An output from the photo-detector 121 is input toan arithmetic circuit 122.

The arithmetic circuit 122 subjects the output from the photo-detector121 to addition/subtraction operations, thereby producing a reproductionsignal corresponding to the information recorded on the optical disk110, a focus error signal, and a tracking error signal. Of thesesignals, the reproduction signal is delivered to a signal processor 123.The focus error signal and tracking error signal are delivered to afocus servo system and a tracking servo system, both not shown.

The signal processor 123 subjects the reproduction signal input from thearithmetic circuit 122 to processing such as equalization, binarization,demodulation and decoding, thus producing reproduction data. Inaddition, the signal processor 123 may have a function of determiningthe kind (track pitch, i.e. recording density) of the optical disk 110on the basis of track pitch data included in physical format datareproduced from a read-in area of the optical disk 110, and outputting adetermination result, as will be described later.

The opening limiting element 118 will now be described.

The opening limiting element 118 is an element for limiting the openingof the objective lens 119 and has a plurality of openings with differentsizes, in particular, dimensions in the radial direction of the opticaldisk 110 (hereinafter referred to as “the disk radial direction”). Theseopenings are switched by an opening switch circuit 124 shown in FIG. 13.The opening switch circuit 124 switches the opening by controlling theopening limiting element 118, for example, in accordance with adiscrimination result from a disk discriminator 125 for determining thekind of the optical disk 110, or a discrimination result 126 of thesignal processing by the signal processor 123.

FIGS. 14A, 14B and 14C show various examples of the structure of theopening limiting element 118.

An opening limiting element, as shown in FIG. 14A, has a rectangularlight shield plate 131 in which a circular opening 132 and an ellipticopening 133 are formed. The elliptic opening 133 has a short axis in thedisk radial direction and a long axis in a track tangential direction onthe optical disk 110. This opening limiting element is parallel-moved bythe opening switch circuit 124 in a direction of double-headed arrow a,and thus one of the openings 132 and 133 is selectively put on the lightincidence path to the objective lens 119 in FIG. 13.

An opening limiting element, as shown in FIG. 14B, has a sectorial lightshield plate 141 in which a circular opening 142 and an elliptic opening143 are formed. The elliptic opening 143 has a short axis in the diskradial direction and a long axis in the track tangential direction. Thisopening limiting element is rotated by the opening switch circuit 124about a rotational axis 140 in a direction of double-headed arrow b, andthus one of the openings 142 and 143 is selectively put on the lightincidence path to the objective lens 119 in FIG. 13.

An opening limiting element, as shown in FIG. 14C, is formed of a liquidcrystal cell 151. An elliptic opening 153 having a short axis in thedisk radial direction and a long axis in the track tangential directionis formed in a central portion of the liquid crystal cell 151. The cell151 is controlled by the turn-on/off of application of voltage to thecell 151 or the magnitude of applied voltage. Specifically, when novoltage is applied to the liquid crystal cell 151, incident light ispassed through the opening 153. When voltage is applied, a circularopening 152 is formed, as indicated by a broken line, and incident lightis passed through the circular opening 152 which includes the opening153. In this case, the control of voltage to the liquid crystal cell 151is performed by the opening switch circuit 124.

In the following description, the openings 132, 142 and 152 shown inFIGS. 14A, 14B and 14C are generally referred to as circular opening A,and the openings 133, 143 and 153 as elliptic openings B. The circularopening A may be the same as the opening of the objective lens 119 andin this case the circular opening A is not needed.

In the optical disk apparatus having the above structure, the beam spotsize on the reflection/recording layer 111 of optical disk 110 isinversely proportional to the numerical aperture (NA) of the objectivelens 119 and proportional to the wavelength of the light beam, i.e.wavelength λ of the semiconductor laser 115 or the light source.Specifically, if the wavelength λ and the numeral aperture NA aredetermined, the shortest pit length representing information on thereflection/recording layer 111 and the optimal value of the track pitchare determined. Accordingly, the optical disk 110 is generally optimizedwith respect to the wavelength λ and the numeral aperture NA.

Since the optical disk 110 is optimized with respect to the wavelength λand the numeral aperture NA, as mentioned above, an optical diskapparatus can basically reproduce information from only the optical diskmatching with the wavelength λ and the numeral aperture NA of thisapparatus. In the present embodiment, however, information can bereproduced from various kinds of optical disks if the opening of theobjective lens 110 is changed by the opening limiting element 118 inaccordance with the kind of the optical disk 110.

The details of the opening limiting function of the opening limitingelement 118 in the optical disk apparatus according to the presentembodiment and the advantages obtained by the opening limiting functionwill now be described.

The degree of modulation M and asymmetry A of the reproduction signalfrom the optical disk 110 which is output from the arithmetic circuit122 are defined as follows.

When the maximum level and minimum level of a reproduction signal Sminof repeat signals of shortest pits are IminH and IminL and the maximumlevel and minimum level of a reproduction signal Smax of repeat signalsof longest pits are ImaxH and ImaxL in FIG. 15, the degree of modulationM and asymmetry A of reproduction signals are defined by $\begin{matrix}{M = \frac{I_{m\quad i\quad n\quad H} - I_{m\quad i\quad n\quad L}}{I_{m\quad a\quad x\quad H} - I_{m\quad a\quad x\quad L}}} & (1) \\{A = \frac{\left( {I_{m\quad a\quad x\quad H} - I_{m\quad a\quad x\quad L}} \right) - \left( {I_{m\quad i\quad n\quad H} + I_{m\quad i\quad n\quad L}} \right)}{2\left( {I_{m\quad a\quad x\quad H} - I_{m\quad a\quad x\quad L}} \right)}} & (2)\end{matrix}$

In general, in the optical disk apparatus, if the degree of modulationof a reproduction signal from the optical disk is M=0.2 or more and theasymmetry thereof is A=−0.05 to 0.15, it is considered that informationcan be correctly reproduced from the reproduction signal. Calculationresults of the degree of modulation M and asymmetry A of reproductionsignals obtained with various combinations of the optical disk apparatusand optical disks will be shown below and it is examined whether suchcombinations meet the above conditions.

One example of the combinations of the optical disk apparatus andoptical disks may comprise an optical disk apparatus (“first opticaldisk apparatus”) having a light source with wavelength λ1=410 nm andhaving an objective lens with numeral aperture NA=0.6, which willpossibly be used as a short-wavelength light source in future, and anoptical disk (“first disk”) with a track pitch=0.42 μm and a shortestpit length=0.23 μm, which will prospectively be used as a so-calledhigh-density DVD.

FIG. 16 shows calculation results obtained with this combination, thatis, calculation results of the repeat reproduction signal of a shortestpit (3T) and the repeat reproduction signal of a longest pit (14T) whichwere reproduced from the first disk by the first optical disk apparatus.

Another example of the combinations of the optical disk apparatus andoptical disks may comprise an optical disk apparatus (“second opticaldisk apparatus”) having a light source with wavelength λ2=650 nm andhaving an objective lens with numeral aperture NA=0.6, which iscurrently used as a modern DVD-ROM system or DVD-RAM system, and anoptical disk (“second disk”) with a track pitch=0.74 μm and a shortestpit length=0.4 μm.

FIG. 17 shows calculation results obtained with this combination, thatis, calculation results of the repeat reproduction signal of a shortestpit (3T) and the repeat reproduction signal of a longest pit (14T) whichwere reproduced from the second disk by the second optical diskapparatus.

TABLE 1 shows in (CASE 1) and (CASE 5) the degree of modulation M andasymmetry A of the repeat reproduction signal obtained from the firstdisk by the first optical disk apparatus and the repeat reproductionsignal obtained from the second disk by the second optical diskapparatus.

TABLE 1 Optical disk apparatus 1st disk apparatus 2nd disk apparatusLight source wavelength 410 nm 650 nm Optical disk 1st disk 2nd disk 2nddisk 2nd disk 2nd disk NA (Radial direction/ (0.6/0.6) (0.6/0.6)(0.36/0.36) (0.36/0.6) (0.6/0.6) tangent direction) Modulation factor0.22  0.78  0.26  0.75  0.30  Asymmetry 0.082 0.207 0.189 0.140 0.017(Case 1) (Case 2) (Case 3) (Case 4) (Case 5)

The degree of modulation M and asymmetry A in (CASE 1) and (CASE 5)satisfy the above conditions, M=0.2 or more and A=−0.05 to 0.15. It isthus considered that exact information reproduction can be performed.

FIG. 18 shows calculation results of the repeat reproduction signal of ashortest pit (3T) and the repeat reproduction signal of a longest pit(14T) which were reproduced from the second disk by the first opticaldisk apparatus. The degree of modulation M and asymmetry A of thereproduction signal in this case are shown in (CASE 1) in TABLE 1. Sincethe degree of modulation M and asymmetry A fail to satisfy the aboveconditions, A=−0.05 to 0.15, good information reproduction cannot beperformed. The reason appears to be that the relationship among thewavelength λ1 of the light source, the beam spot size on thereflection/recording layer of the optical disk, and the shape of pits isnot proper.

In the present embodiment, the condition of asymmetry is not satisfied.However, there may be a case where the condition of a modulation factoris not satisfied, depending upon the combination between an optical diskapparatus and an optical disk or a case where the conditions of both themodulation factor and the asymmetry are not satisfied.

FIG. 19 shows calculation results of the repeat reproduction signal of ashortest pit (3T) and the repeat reproduction signal of a longest pit(14T) which were reproduced from the second disk, which is the opticaldisk 100, with use of the first optical disk apparatus wherein thenumerical aperture NA of the objective lens was set at 0.36 so that thebeam spot size may become equal to that in the second optical diskapparatus. In this case, the degree of modulation M and asymmetry A ofthe reproduction signal are shown in (CASE 3) in TABLE 1 and fail tosatisfy the above condition, A=−0.05 to 0.15. Thus, good informationreproduction cannot be performed.

However, if the opening of the objective lens 119 is limited fromcircular opening A to elliptic opening B by the opening limiting element118 in the optical disk apparatus (first optical disk apparatus)according to the present embodiment shown in FIG. 13, the beam spotshape on the reflection/recording layer 111 can be optimized. Thus, thereproduction signal capable of exactly reproducing information from thesecond disk can be obtained.

FIG. 20 shows calculation results of the asymmetry A of the reproductionsignal reproduced from the second disk with use of the optical diskapparatus (first optical disk apparatus) shown in FIG. 13 wherein thenumerical aperture NA in the disk radial direction and the numericalaperture NA in the track tangential direction of the objective lens 119were varied. It is understood from FIG. 20 that a hatched region, wherethe NA in the disk radial direction is 0.396 or less and the NA in thetrack tangential direction is 0.57 or more, satisfies the condition,A=0.05 to 0.15. In other words, it is understood that where informationis reproduced from the second disk by the first optical disk apparatus,the asymmetry is improved as the NA in the disk radial direction isdecreased and the NA in the track tangential direction is increased.

When the objective lens 119 is mass-produced, it is generally difficultto produce lenses with the numerical aperture NA=0.66. It is thusdesired that the upper limit of the NA in the track tangential directionbe set at 0.66. In addition, if the NA in the disk radial direction isdecreased, the beam spot size in the disk radial direction increases andleak from adjacent tracks increases. It is thus desired that when the NAin the disk radial direction of the objective lens 119 is limited by theopening limiting element 118, the lower limit of the NA be set at 0.35in order to make the beam spot size substantially equal to that in thecase of reproducing information from the second disk by the secondoptical disk apparatus.

In brief, when information is reproduced from the second disk by thefirst optical disk apparatus, the numerical apertures NA2r and NA2t inthe disk radial direction and track tangential direction, with which thereproduction signal capable of exactly reproducing information, aregiven by

0.35≦NA2r≦0.396  (3)

0.57≦NA2t≦0.66  (4)

FIG. 21 shows calculation results of the reproduction signal of ashortest pit (3T) and the repeat reproduction signal of a longest pit(14T) which were reproduced from the second disk, which is the opticaldisk 100, with use of the optical disk apparatus (first optical diskapparatus) shown in FIG. 13 wherein the elliptic opening B withnumerical aperture NA=0.36 in the disk radial direction and numericalaperture NA=0.6 in the track tangential direction is set on the lightincidence path to the objective lens 119. In this case, the degree ofmodulation M and asymmetry A of the reproduction signal are shown in(CASE 4) in TABLE 1 and satisfy the conditions, M=0.2 or more andA=−0.05 to 0.15. It is understood from these results that exactinformation reproduction can be performed by setting the beam spot shapewith use of the proper opening limited by the opening limiting element118.

Specifically, the influence of asymmetry due to the pit depth can bereduced by increasing the beam spot size in the disk radial directionand thus increasing the amount of beam radiation on an area other thanthe pit portion. Besides, since the optical resolution is enhanced bythe reduction in beam spot size in the track tangential direction, theamplitude ratio (modulation factor) of a signal with highest density toa signal with lowest density can be increased and the influence ofasymmetry can be reduced.

Moreover, since the beam spot size is proportional to the wavelength andinversely proportional to the numerical aperture NA, formulae (3) and(4) can be rewritten to the following formulae (5) and (6) by using thewavelength (λ) of the light source and the numerical aperture (NA) ofthe objective lens: $\begin{matrix}{{{NA1r} \cdot \frac{410}{650} \cdot 0.95} \leqq {NA2r} \leqq {{NA1r} \cdot \frac{410}{650} \cdot 1.1}} & (5)\end{matrix}$

 NAlt·0.95≦NA2t≦NAlt·1.1  (6)

where $\begin{matrix}{{{{NA1r} \cdot \frac{410}{650}} \cong 0.36},{{NA1t} \cong 0.6}} & (6)\end{matrix}$

In brief, by establishing the relationships of formulae (5) and (6) bymeans of the opening limiting element 118, the reproduction signalcapable of exactly reproducing information can be obtained from thesecond disk with use of the first optical disk apparatus.

The above description may be summarized as follows.

In the optical disk apparatus according to the present invention, thewavelength λ1 of the semiconductor laser 115 is constant and the openingof the objective lens 119 can be switched by the opening limitingelement 118 between the circular opening A with numerical aperture NAlrin the disk radial direction and numerical aperture NAlt in the tracktangential direction and the elliptic opening B with numerical apertureNA2r in the disk radial direction and numerical aperture NA2t in thetrack tangential direction. Suppose that the degree of modulation andasymmetry of the reproduced signal, which is obtained from the firstdisk by the first optical disk apparatus having the light source withwavelength λ1 and the objective lens with numerical aperture NAlr in thedisk radial direction and numerical aperture NAlt in the tracktangential direction, satisfy the above-mentioned conditions, M=0.2 ormore and A=−0.05 to 0.15, and that the degree of modulation andasymmetry of the reproduced signal, which is obtained from the seconddisk by the second optical disk apparatus satisfy the same conditions.At this time, if the wavelength and numerical aperture (NA) meet thefollowing relationship, $\begin{matrix}{{0.95 \cdot \frac{\lambda 1}{\lambda 2} \cdot {NA1r}} \leqq {NA2r} \leqq {1.1 \cdot \frac{\lambda 1}{\lambda 2} \cdot {NA1r}}} & (7)\end{matrix}$

 0.95·NAlt≦NA2t≦1.1·NAlt  (8)

a reproduction signal capable of exactly reproducing information can beobtained from each of the first and second disks by selecting, in theoptical disk apparatus of this embodiment, the circular opening A in thecase of reproducing information from the first disk or the ellipticopening B in the case of reproducing information from the second disk.

In fact, if the numerical apertures NAlr and NAlt are set to be equal tothe numerical aperture NA of the objective lens 119 in the disk radialdirection and track tangential direction (NA=NAlr=NAlt), the openinglimiting element 118 may not have the circular opening A, as mentionedabove. When information is to be reproduced from the first disk, theopening limiting element 118 may be set in such a state as to pass allthe beam. When information is to be reproduced from the second disk, theelliptic opening B may be set on the light path to the objective lens.

The optical disk apparatus capable of reproducing information from twokinds of optical disks (first and second disks) with different recordingdensities has been described. The present invention, however, isapplicable to reproduction of information from three or more kinds ofoptical disks wherein the pit length (in particular, minimum pitlength), track pitch, etc. are optimized with respect to different lightsource wavelengths. In this case, it should suffice to use the openinglimiting element 119 capable of switching three or more sets ofnumerical apertures in the disk radial direction and track tangentialdirection, including the numerical aperture of the objective lens 119itself.

The opening limiting element for limiting the opening of the objectivelens switches the opening in accordance with the optical disk to bereproduced, thereby optimizing the beam spot shape. Therefore,information can be exactly reproduced from plural kinds of optical diskswith different recording densities.

Specifically, the optical disk apparatus with the light sourcewavelength of, e.g. 410 nm, can exactly reproduce information fromeither a relatively high-density optical disk optimized to match withthis wavelength or a relatively low-density optical disk optimized atlight source wavelength of 650 nm.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. An optical disk apparatus comprising: a pick-upunit including a light source, an optical system which focuses a lightbeam emitted from the light source onto an optical disk, and aphoto-detector which detects reflection light from the disk, the pick-upunit reproducing information recorded on the disk along a predeterminedtrack; and an opening limiting unit which limits an opening of theobjective lens such that numerical apertures NAlr and NAlt in a diskradial direction and a track tangential direction of the objective lensin a case where said optical disk is a first disk optimized with respectto a wavelength λ1 of the light source, and numerical apertures NA2r andNA2t in the disk radial direction and the track tangential direction ofthe objective lens in a case where said optical disk is a second diskoptimized with respect to a wavelength λ2 (λ1<λ2) of the light source,satisfy formulae:${{0.95 \cdot \frac{\lambda 1}{\lambda 2} \cdot {NA}}\quad 1r} \leqq {{NA}\quad 2r} \leqq {{1.1 \cdot \frac{\lambda 1}{\lambda 2} \cdot {NA}}\quad 1r}$

 0.95·NAlt≦NA2t≦1.1·NAlt.
 2. The optical disk apparatus according toclaim 1, wherein the first disk has a track pitch of 0.42 μm and aminimum pitch length of 0.23 μm, the second disk has a track pitch of0.74 μm and a minimum pitch length of 0.4 μm, and said λ1, λ2, NAlr,NAlt, NA2r and NA2t are, respectively, λ1=390 nm to 420 nm, λ2=650 nm to780 nm, NAlr=NAlt=0.6, NA2r=0.33 to 0.4, and NA2t=0.54 to 0.66.
 3. Theoptical disk apparatus according to claim 1, wherein said openinglimiting unit is formed of a plate-like member with at least oneopening, which plate-like member can be set on a light incident path tothe objective lens.
 4. The optical disk apparatus according to claim 1,wherein said opening limiting unit is formed of a liquid crystal cellwith at least one opening, a size of which is varied by application ofvoltage.
 5. The optical disk apparatus according to claim 1, whereinsaid opening limiting unit has at least an elliptic opening having ashort axis in the disk radial direction and a long axis in the tracktangential direction.
 6. An optical disk apparatus comprising: a pick-upunit including a light source, an objective lens which focuses a lightbeam emitted from the light source onto an optical disk, and aphoto-detector which detects reflection light from the disk, the pick-upunit reproducing information recorded on the disk along a predeterminedtrack; and an opening limiting unit which limits an opening of theobjective lens such that numerical apertures in a disk radial directionand a track tangential direction of the objective lens are made equal ina case where said optical disk is a first disk having a predeterminedrecording density and optimized with respeact to a wavelength λ1 of thelight source, and the numerical aperture in the disk radial direction ofthe objective lens is made less than that in the track tangentialdirection of the objective lens in a case where said optical disk is asecond disk having a recording density lower than the first disk andoptimized with respect to a wavelength λ2 (λ1<λ2) of the light source,wherein said opening limiting unit has at least an elliptic openinghaving a short axis in the disk radial direction and a long axis in thetrack tangential direction.
 7. An optical disk apparatus comprising: apick-up unit including a light source, and objective lens which focusesa light beam emitted from the light source onto an optical disk, and aphoto-detector which detects reflection light from the disk, the pick-upunit reproducing information recorded on the disk along a predeterminedtrack; and an opening limiting unit which limits an opening of theobjective lens such that a beam spot shape on the optical disk is thesame in a disk radial direction and a track tangential direction of theoptical disk in a case where said light source has a wavelength λ1 andsaid optical disk is a first disk having predetermined recordingdensity, and the beam spot shape on the optical disk is large in thedisk radial direction and small in the track tangential direction in acase where said optical disk is a second disk having a recording densitylower than the first disk and optimized with respect to a wavelength λ2(λ1<λ2) of the light source, wherein said opening limiting unit has atleast an elliptic opening having a short axis in the disk radialdirection and a long axis in the track tangential direction.